In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and the DC voltage of the DC power source is converted into AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in the hybrid automobile and the electric automobile like this, there is known the following technology: that is, a current sensor for sensing a phase current is disposed in only one phase to thereby decrease the number of current sensors, which reduces a space near three-phase output terminals of an inverter in dimension and hence reduces also the cost of a control system of the AC motor. In the case where the current sensor is disposed in only one phase, for example, in patent document 1, three-phase AC current command values are calculated on the basis of a current sensed value of the one phase (hereinafter referred to as “sensor phase”) for sensing a phase current by means of the current sensor, a d-axis current command value and a q-axis current command value, and an electric angle of the AC motor and are estimated as current values of phases for each of which a current sensor is not disposed (hereinafter referred to as “estimated phase” as required).
For example, in the case where the control device of the AC motor is mounted in the hybrid automobile or the electric automobile as described above, when an accelerator pedal or a brake pedal is depressed, in some cases, a torque command value is suddenly changed by a user (man) request. Further, in the case of performing a traction control system for preventing a tire from being idled according to the state of a road surface or the like, in some cases, the torque command value is suddenly changed by a vehicle request. When the torque command value is suddenly changed, a d-axis current command value and a q-axis current command value, which are determined on the basis of the torque command, are also suddenly changed.
In an estimation method of the patent document 1, for example, in the case where the d-axis current command value and the q-axis current command value are suddenly changed, when the d-axis current command value and the q-axis current command value are used for estimating a current of an estimation phase for which the current sensor is not disposed as described above, a current estimated value of the estimation phase is also suddenly changed by a sudden change in the d-axis current command value and the q-axis current command value.
In particular, in the case where the update cycle of the d-axis current command value and the q-axis current command value is longer than an operation cycle of a voltage command value, when the current estimated values calculated on the basis of the d-axis current command value and the q-axis current command value in each of which an update cycle is long are fed back to thereby calculate a voltage command value, the voltage command value is also suddenly changed and hence voltage to be impressed on the AC motor is suddenly changed.
Further, depending on the type and construction of control, in some cases, current sensing by the current sensor is performed in a short cycle, and the update of the d-axis current command value and the q-axis current command value and the operation of a voltage command value by current feedback are performed in a longer cycle than a current sensing cycle by the current sensor. Even in the case where the current estimated value calculated on the basis of the current command value is fed back and the voltage command value is calculated in a slow cycle, a variation range of the voltage command value is increased by a sudden change in the current command value and hence when the voltage command value is updated, voltage to be impressed on the AC motor is likely to be suddenly changed.
When the voltage to be impressed on the AC motor is suddenly changed, an excessive overshoot or undershoot is caused in the output torque outputted from the AC motor and hence a desired torque cannot be outputted, which hence could make the drive control of the AC motor unstable. However, the patent document 1 does not take any countermeasure for the case where the d-axis current command value and the q-axis current command value are suddenly changed in this manner.    [Patent document 1] JP-A-2008-86139 (corresponding to US 2008/0079385)